dogs and cats have been bred and domesticated over hundreds (if not thousands) of years, to get to the point they are now. fish however, are just fish. with no discernible difference bewtween a wild caught fish and ones bred in shops. obviously there is quite a difference between wild dogs and your fluffy little yorkie terrier that lays claim to the couch.
point is, the dogs and cats we have now are bred for homelife, where ever that home is - obviously you need to select your dog based on the level of care/space you can give it. with fish, particuarly wild caught fish, you could argue its cruel. to say 'but the fish wont get eaten in my tank", i dont go with it. survival of the fittest, every animal has its place in the food chain for a reason.